The invention relates to superplastic forming and diffusion bonding and particularly to an improved apparatus and method for making laminate structures from a plurality of worksheets.
"Superplasticity" is the property of a material to develop unusually high tensile elongations with a reduced tendency toward necking. This property is only exhibited by a limited number of metals. Titanium and its alloys, and certain aluminum alloys are examples of metals that exhibit superplasticity.
The process of superplastic forming and diffusion bonding has been disclosed in U.S. Pat. No. 3,927,817 by Hamilton et al., entitled "Method for Making Metallic Sandwich Structures", which is incorporated into this specification by reference. Generally, the patent discloses a method of making diffusion bonded and superplasticity formed sandwich structures. The forming tool includes a first fluid line and the stacked worksheets include a second fluid line. The worksheets are coated with a stopoff material in selected areas that are not to be diffusion bonded. The uncoated areas of the stacked worksheets are diffusion bonded together by applying a pressure differential, and at least one of the blanks is superplastically formed against at least one of the die surfaces forming a sandwich structure. Expansion tubes are inserted into a recess in the stack and are used to insure even gas flow into the worksheets during superplastic forming.
Considerable time is required to make the recess and to install the expansion tubes. The tubes are typically made of steel and the high diffusion bonding and superplastic forming pressures may crush the bore in the expansion tube.
U.S. Pat. No. 4,331,284 by Schulz et. al., entitled "Method of Making Diffusion Bonded and Superplastically Formed Structures", which is also incorporated into this specification by reference, discloses a method of simplifying the tooling. A hollow tubular member is inserted into the stacked worksheets and coupled to the pressurized inert gas line. The member extends to the stopoff path, thereby eliminating the need for the expansion tubes and the recess in the stacked sheets. However, this latter approach imposes some other problems. The worksheets are preferably Ti-6Al-4V, and when steel tubular members are used, the nickel in the steel tubes reacts with the titanium in the worksheets causing contamination in the interior surface of the formed part. While the expansion tubes are easily removed from the formed structure, the hollow tubes are not and the contamination problem is extended. In addition, when the hollow tube is thicker than the core worksheet, grooves are needed in the bottom forming die and in the bottom face sheet that is positioned under the hollow tube. The grooves allow the hollow tube to fit into the core sheet while minimizing the compressive forces applied to the hollow tube. However, the grooves unduly complicate the forming process, and require precise worksheet alignment. The face sheet groove is removed by subsequent machining.
U.S. Pat. No. 4,304,350 by Paez et. al., entitled "Method of Pressurization System For Superplastic Forming and Diffusion Bonding" discloses yet another system for forming laminate structure by superplastic forming and diffusion bonding. A hollow stainless steel pin is inserted into a recess in one die member and into an opening in the bottom face sheet perpendicular to the sheet. The needle is a complex shape requiring additional machining within tight tolerances. The tooling design is further complicated when three or more worksheets are involved. Also, a stainless steel pin can cause contamination.
A new forming apparatus and method is needed for forming sandwich structures using superplastic forming and diffusion bonding which overcomes the disadvantages of the prior art.